Satellite television receivers generally include a low noise block converter (LNB) at the satellite dish for controlling reception of satellite television signals. Functions of the LNB include downconverting received satellite signals, changing the frequency band of signal reception, changing the signal polarization of reception and, in some applications, controlling more than one receive antenna. For these purposes, the LNB requires control signals and power, both of which are provided by circuitry housed in the box located near the television set through which a user can change the channel of reception (sometimes referred to as the “set top box”). Since only a single coaxial cable couples the LNB to the set top box, the control and power signals are carried by the single coaxial cable.
EUTELSAT, a European organization which governs television satellite communications, establishes specifications for the low noise block converter control and power signals. Analog AC tone control signals are provided by a 22 kHz, 600 mV peak-to-peak signal which can be used to implement DiSEqC™ (Digital Satellite Equipment Control) encoding for the purpose of changing the polarization and frequency band of received RF signals. Digital control information which is also used to change the frequency band of received RF signals is transmitted as a nominal 13 volt DC signal for a logic zero and as a nominal 18 volt DC signal for a logic one. In practice however, a logic zero corresponds more broadly to voltages between approximately 12–14 volts and a logic one corresponds to voltages between approximately 17–20 volts. Power to the LNB circuitry itself is provided by whatever DC voltage is being used to transmit digital control information at any given time. Further, the LNB circuitry requires on the order of 0.6 amp of current.
Adjustable linear amplifiers are conventionally used to provide the control and power signals to the LNB. In this arrangement, the linear amplifier provides an AC modulated output voltage which has an adjustable DC voltage level in order to provide a logic one voltage level or a logic zero voltage level in response to control signals from a microprocessor within the set top box. In order to provide the logic one voltage levels, the minimum input voltage to the linear amplifier is required to be on the order 20 volts. However, when a logic zero voltage level of 12 volts is selected, the resulting power dissipation of the linear amplifier approaches 5 watts.
One attempt at lowering the power dissipation associated with the use of a linear amplifier to provide power and control signals to an LNB is the LNBP10 Series LNB Supply and Control Voltage Regulator product of ST Microelectronics of Lincoln, Mass. In this product, two input voltage levels are available to the linear amplifier, depending on which output voltage level is selected. The input voltage levels are provided by an unregulated AC/DC converter.
While the ST product somewhat reduces power dissipation as compared to the conventional linear amplifier arrangement, use of the ST product requires two windings on the power transformer of the AC/DC converter, only one of which is in use at any given time, thereby adversely impacting the efficiency of the converter. Further, even with two input voltage levels, the voltages dropped across the linear amplifier are still significant. As a result, power dissipation is still high enough to require relatively large package dimensions due to power dissipation and thermal considerations.